1. Field of the Invention
This invention relates to a method of making a silicon diode array target for camera tubes and more particularly to a simplified technique of providing closely spaced conductor pads that contact the diodes for improved beam landing.
2. Description of the Prior Art
Among the types of targets that are used in vidicons are photodiode array targets such as are described in U.S. Pat. No. 3,548,233 to Cave et al, issued on Dec. 15, 1970. Generally, such targets include a semiconductor wafer with a bulk region of one conductivity type and have an array of discrete regions of another conductivity type on one of its major surfaces. The discrete regions form PN junctions with the bulk region of the wafer. The bulk region surface separating the discrete regions is covered completely with an insulating layer. A semiconductor pad covers and is in electrical contact with each discrete region. These pads increase the amount of landing area for the discrete regions and decrease the amount of exposed insulating area, to result in improved beam landing.
The current procedure for fabricating a silicon diode array target is to initially define the diode openings in an insulating area by using a non-contacting photolithographic process followed by a chemical vapor deposition of "P" doped polycrystalline silicon that is covered with a masking oxide to be used for conductor pad definition. The established procedure for defining the conductor pads is to use contact printing techniques with a redundant overlay alignment and etching cycle to obtain the close spacing separating conductor pads and also to reduce the number of defects in the pads. This requires two photoresist coatings and curing cycles, two alignment cycles, two developing and baking cycles, two etching and photoresist striping cycles, plus several additional chemical cleaning steps. These cycles are very costly in both labor and material. Moreover, each contact of the photomask with the photoresist is likely to cause defects such as scratches or adhering opaque foreign matter on the photomask. These defects in the photomask usually result in defective diodes in the finished target which are quite noticable in a displayed signal from the target. The accumulation of defects in a photomask also severely limits its useful lifetime, and the replacement of defective highdensity array masks is a significant cost factor in the production of silicon vidicon targets.
Projection photolithographic techniques have also been utilized to define the conductor pads as disclosed in U.S. Pat. No. 3,615,449 to Greenaway. However, the spacing between conductor pads cannot be obtained as finely with such projection techniques as with the contact printing and have generally not been utilized where maximum conductor pad area is desired.